By the turn of the century it was well known that an emission
spectrum existed for the various elements. The experiment is
easy: An element, such as hydrogen, is excited by an electrical
potential until it gives off light. The emitted light is then
passed through a prism - which separates it into its constituent
wavelengths. The wavelengths are then recorded on light sensitive
film.

Each element gives a unique spectrum. The position of the
lines relative to one another changes from element to element
and, as it turns out, from molecule to molecule. In essence,
the emission spectrum is a fingerprint of the element/molecule
that generates it.